1. Technical Field
This disclosure relates to a multilayer phase-change information recording medium used for performing any one of recording and reproducing of information by means of laser beam irradiation, and a method for optical recording and reproducing using the multilayer phase-change information recording medium.
2. Description of the Related Art
A phase-change optical disc such as CD-RW (Compact Disc-Rewritable) (hereafter this may be referred to as “phase-change information recording medium”, “optical information recording medium”, and “optical recording medium”) generally has the basic configuration in which a recording layer comprising a phase-change material is provided on a plastics substrate, and a reflective layer which improves the light absorbance and has thermal diffusion effect is formed on the recording layer to perform recording and reproducing of information by means of laser beam irradiation from the substrate side.
The above-noted phase-change material induces a phase change between crystalline conditions and amorphous conditions by heating through the use of laser beam irradiation and subsequent cooling, and after the material is rapidly heated, if quenched, the material becomes amorphous, and if slowly cooled, the material becomes crystalline. Phase-change information recording media apply the properties to recording and reproducing of information.
Also, a phase-change information recording medium usually comprises a lower protective layer (hereafter this may be referred to as “lower dielectric layer”) provided between a substrate and a recording layer, and an upper protective layer (hereafter this may be referred to as “upper dielectric layer”) provided between the recording layer and a reflective layer for preventing oxidation, transpiration, and deformation of the recording layer, which are caused by heating through laser beam irradiation. In addition, these protective layers respectively have a function for adjusting optical properties of the information recording medium by adjusting the thicknesses thereof. Further, the lower protective layer also has a function for preventing the substrate from getting soft due to heat induced at the time of recording to the recording layer.
With increases in information volume handled with computers and the like in recent years, the signal recording capacity of optical discs such as DVD-RAM (Digital Versatile Disc-Random Access Memory), DVD+RW (DVD-Rewritable) has increased, which is accelerating high-density recording of signal information. The recording capacity of CD is currently approx. 650 MB, and DVD currently has a storage capacity of approx. 4.7 GB. Further increases in demands for high-density recording can be expected. Examples of the methods for achieving high-density recording using such a phase-change information recording medium include shortening the used laser beam wavelength up to blue ray region or increasing the numerical aperture (NA) of the objective lens to be used for pickup in recording and reproducing to reduce the spot size of laser beam irradiated to the information recording medium.
As a method for increasing recording capacity of an optical disc by means of improving an information-recording medium itself, a bilayer phase-change information-recording medium has been proposed, which is produced by laminating information layers, each of which comprising at least a recording layer and a reflective layer, on single-sided of the substrate and by bonding these information layers with an ultraviolet curable resin and the like therebetween. (For instance, see Japanese Patent (JP-B) No. 2702905, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-215516, 2000-222777, 2001-243655, and the like).
A separating layer which is the interfacing portion between two information layers (it may be referred to as “intermediate layer”) has a function for separating two information layers optically and comprises a material that absorbs laser beam as little as possible, because laser beam used in recording and producing needs to reach as the innermost information layer as possible.
Any of the above-noted bilayer phase-change information recording media are characterized by the first information layers thereof, and a first protective layer and a second protective layer are provided in the first information layer, as in the case of single-layer phase-change information recording media. These bilayer phase-change information recording media are also disclosed on the page 22 of “Phase Change Material for Use in Rewritable Dual-layer Optical Disk Utilizing a Blue-violet Laser”—ODS2001 Technical Digest, but there are many problems with such media. For example, if an information layer disposed at the near side from the laser beam irradiation side (the first information layer) is not sufficiently transmitted by laser beam, it will be impossible to record information in the recording layer of the information layer disposed at the innermost side (the second information layer) and to reproduce the information. To solve this problem, it is conceivable to remove a reflective layer constituting the first information layer, to make a reflective layer ultrathin, or to form a recording layer constituting the first information layer ultrathin.
Recording by the above-noted phase-change information recording media is performed by means of laser beam irradiation to the phase-change material of the recording layer and by quenching the material to induce transformation from crystalline conditions to amorphous conditions then to form a mark. Therefore, there is a problem that if the reflective layer is removed or is made very thin up to around 10 nm, it becomes difficult to form an amorphous mark due to the reduced thermal diffusion effect. Especially, a Sb—Te eutectic recording material, which is one of the materials generally used for phase-change information recording media such as CD-RW, is known as an excellent material in that the outline of amorphous portion of a recording mark is clear, because the material has a higher erasing ratio and a higher recording sensitivity than a Ge—Sb—Te compound recording material.
However, in order for the above-noted Sb—Te eutectic recording material to be transformed to amorphous, the material needs to have a quenching configuration in which quenching in a shorter time is required, because the crystallization rate of Sb—Te eutectic recording material is faster than a Ge—Sb—Te compound recording material. Therefore, a Sb—Te eutectic recording material has a problem that forming a mark will be difficult with the configuration having a thin reflective layer.
On the other hand, a method for further providing a layer for assisting in the thermal diffusion function that has been carried out by a reflective layer (this may be referred to as “thermal diffusion layer”) on the reflective layer using a nitride, a carbide or the like having a relatively greater thermal conductivity and a smaller light absorbance, was proposed for a single-layer phase-change information recording medium (Japanese Paten Application Laid-Open (JP-A) No. 08-50739) and a bilayer phase-change information recording medium (JP-A No. 2000-222777), and the like.
This method can be considered as an effective method for eliminating the above-mentioned disadvantages that occur when forming a reflective layer constituting the first information layer very thin.
However, these materials like nitride or carbide have a higher stress, so a formed thermal diffusion layer is subject to cracks. Consequently, this develops a problem that the optical disc itself with a thermal diffusion layer provided therein cannot get sufficient overwrite properties. In addition, in a next-generation optical disc such as Blue-ray Disk system which uses a blue-violet laser, a carbide material has a problem that it is impossible to increase the light transmittance of the first information layer, because a carbide has a high light absorbance particularly on the short-wavelength side.
Hence, a multi-layer phase-change information recording medium capable of keeping a high light transmittance even in the short-wavelength region, allowing a recording layer with three layers provided therein, and having the improved sensitivity even in the case of two-layer optical information recording medium, have not been is provided yet, and immediate presentation of such a recording medium is desired under the present situation.